While not necessarily limited thereto, the present invention is particularly adapted for use as a rectrofit kit for older-type model trains. The whistle used on a model steam engine of this type was ordinarily actuated by a shaded pole relay which, in turn, energized an alternating-current whistle motor. In diesel engine models, a direct-current buzzer, powered by a battery, was also actuated by a shaded pole relay. The relay, in turn, was tripped by a direct-current voltage superimposed on the alternating-current voltage used to power the engine.
In prior art devices of this type, the shaded pole relay had a tendency to become magnetized over a period of time, holding the relay contacts closed. In the case of a battery-operated horn, this condition would, of course, drain the battery; and, in addition, the battery contacts would often become corroded or bent, making the device inoperable.